1. Field of the Invention
The invention relates to an apparatus for correcting self-alignment torque of a vehicle, and to an electric power steering apparatus furnished with a self-alignment torque correcting function.
2. Description of Related Art
The steering mechanism of a vehicle is designed to produce self-alignment torque that forces the self into the straight steering state against a reaction force applied from the road surface. In other words, when the driver moves his hands off the steering wheel while the vehicle is traveling, the vehicle tries to travel in the straight direction. The vehicle characteristic would be ideal if such a traveling characteristic can be achieved in the entire speed range (except for the engine stopped state) and at all the steering angle positions.
However, in some vehicles, when the driver moves his hands off the steering wheel while the vehicle is traveling in a specific speed range (generally, in the low speed range) at a specific steering angle (large steering angle), the steering mechanism is not forced into the straight steering state; on the contrary, it is steered to the right or the left fully to the limit.
For example, as shown in FIG. 3, the self-alignment torque takes a positive value (in the direction forcing the steering mechanism into the straight steering state) in the high and medium speed ranges at all the steering angle positions, whereas the self-alignment torque may take a negative value in the low and extremely low speed ranges at large steering angles.
According to one of the related-art self-alignment torque correcting techniques, a spring is linked to the rack axis and the steering mechanism is returned to the straight steering state by a force of the spring. However, because the ability of a vehicle to travel in the straight direction is maintained by a force of the spring regardless of the speed range, in the case of a vehicle having a problem in the ability to travel in the straight direction only in a specific speed range, the self-alignment torque may be corrected as well in the other speed ranges that need no correction.
In addition, a problem in the aforementioned related-art technique that corrects the self-alignment torque mechanically is that because the self-alignment torque correcting structure using a spring has to be designed differently for each type of vehicle, the number of man-hours needed to develop the structure is increased markedly.